The spacecraft is billions of miles beyond the orbits of the planets in our solar system, but it is still within the system, continuing to detect solar winds, or electrically charged gases ejected from our sun.

Based on new data from the craft, NASA announced last week that Voyager 1 was nearing the "heliopause," which scientists believe is the border between our solar system and interstellar space, says NASA research scientist Eric Christian in Greenbelt, Md.

The heliopause is the point where solar winds stop and magnetic fields shift from the solar system to that of deep space.

"The latest data indicate that we are clearly in a new region where things are changing more quickly," Stone says.

Voyager 1 and its identical sister ship, Voyager 2, also launched in 1977, between them explored all of the solar system's outer planets of Jupiter, Saturn, Uranus and Neptune in the 1980s.

Back in 2004, the spacecraft crossed a boundary called the "termination shock," where the solar winds slow as they first meet interstellar space, which some scientists thought was the edge of the solar system.

Where exactly the solar system "ends" is an ongoing debate among scientists. Voyager provides answers as it traverses this unexplored region.

"The most surprising thing about the outer limits of the solar system is how dynamic it is," Christian says. "Even though the outer solar system is an extremely good vacuum, there is still a lot going on. We're going where no one has been before."

Both Voyagers continue to make unexpected discoveries about the physical structure of the solar system.

"We've been surprised by Voyager again and again," Christian says. "I actually think that the Voyager mission is one of NASA's best success stories. That both Voyagers are still working well, nearly 35 years after launch, and sending back important science from 10 billion miles away is truly amazing."

In about 200,000 years or so, one of the craft might come close to another star, Christian says.

Though they will lose the power to transmit data back to Earth sometime between 2020 or 2025, both will continue their journey into deep space.